In perpendicular magnetic recording (PMR) media, increases in areal density have been accomplished by scaling down the area of a data bit by reducing the grain size of the magnetic particles in the recording medium. In order to maintain thermal stability in the smaller grains, media with higher magnetic anisotropy is used. A higher magnetic anisotropy makes the magnetization more resistant to thermal fluctuations. However, a higher magnetic anisotropy also means that a stronger writing field is required to write a data bit. Due to the limit of the writing field a recording head can induce, media with high magnetic anisotropy may exceed a coercivity threshold of the writing field and may not be recordable by conventional recording heads.
One way to decrease the writing field of high anisotropy media is to use an exchange spring media, also known as exchange coupled composite (ECC) media. FIG. 1 illustrates a single ECC layer media structure 100 in which a composite recording layer employs a magnetic soft layer 103 to exchange couple a magnetic hard layer 101 below. In such single ECC layer media structure 100, the magnetic soft layer 103 will switch in presence of an external field and apply a magnetic torque to assist in the switching of the magnetic hard layer 101 below, and thereby decreases the writing field required to switch the magnetization of the highly anisotropic media. To provide better control of the exchange coupling, an exchange coupling layer 102 is disposed between the magnetic hard layer 101 and the magnetic soft layer 103. By changing the thickness of the exchange coupling layer 102, the degree of exchange coupling can be adjusted. The exchange coupling layer 102 also enables epitaxial transfer of texture from the hard magnetic recording layer 101 to the soft magnetic recording layer 103. U.S. Pat. No. 7,572,526 describes a single ECC layer media structure with a hard magnetic recording layer at the bottom and a multilayer soft layer on top. While such a structure can reduce the writing field required for highly anisotropic media, the reduction is limited because the assisting magnetic torque is only applied from the top side of the magnetic hard layer.